404 ON THE CHANGE OF REFRANGIBILTTY OF LIGHT. than double that of p ; so that the length of the spectrum, reckoned from Hy was more than double the length of the part previously known from photographic impressions. The light was reflected by the metallic speculum of a Silbermann's heliostat, which I have received from M. Duboscq-Soleil. With the glass train the groups was faint, but with the quartz train there was abundance of light to see not only the group p, but the fixed linos as far as Hpl, or thereabouts. From the group n to about the middle of the new region, the lines are less bold and striking than in the region of the groups //, I, m, n, but the latter part of the new region contains many lines remarkable both for their strength and for their arrangement. I hope to make a careful drawing of these lines as shown by the complete train with a summer's sun. I have some reasons for believing that the photographic action of these highly refrangible rays is feeble, perhaps almost absolutely null. In the second of the papers referred to in Note A (p. ;$()()), M. Becquerel describes an experiment in which a prism of quartz was employed to form a spectrum ; and yet the impressed spectrum formed by rays which had traversed the quartz alone was hardly longer than that formed by rays which, in addition to the quartz, had traversed a screen of pure flint-glass a centimetre in thickness. It is possible, I am inclined to think probable, that glass made with perfectly pure materials would be transparent like quartz, but all the specimens I have examined were decidedly defective in transparency. Besides, M. Becquerel, who may be allowed to be the best judge of his own experiments, considered the result just mentioned as a proof that the impressed spectrum formed by rays which had traversed quartz only did not extend, except a very trifling distance, beyond that formed by his train of glass; and yet his map, formed by means of the latter, does not take in the line p. However, among the multitude of preparations capable of being acted on by light, it is probable that there may he some which are acted on mainly by rays of unusually high refran^ibilif y, and which, on that very account, would not be suitable for the ordinary purposes of photography. With these it. is possible that the new region of the solar spectrum might he taken photograp hical ly.